1. Field of the Invention
The present invention relates to a driver circuit including an element that is formed using a metal oxide exhibiting semiconductor characteristics, and to a semiconductor device using the driver circuit. Note that the semiconductor device indicates all the devices that can operate by using semiconductor characteristics, and display devices, semiconductor circuits, and electronic appliances are all included in the category of the semiconductor devices.
2. Description of the Related Art
A wide variety of metal oxides exist and are used for various applications. Indium oxide is a well-known material and is used as a transparent electrode material needed for a liquid crystal display and the like.
Some metal oxides exhibit semiconductor characteristics. Metal oxides exhibiting semiconductor characteristics are a kind of compound semiconductor. The compound semiconductor is a semiconductor obtained by bonding two or more kinds of atoms. In general, metal oxides are insulators; however, it is known that metal oxides become semiconductors depending on the combination of elements included in the metal oxides.
For example, it is known that some metal oxides such as tungsten oxide, tin oxide, indium oxide, and zinc oxide exhibit semiconductor characteristics. References disclose a thin film transistor in which a transparent semiconductor layer including such a metal oxide is used as a channel formation region (Patent Documents 1 to 4, and Non-Patent Document 1).
As metal oxides, multi-component oxides as well as single-component oxides are known. For example, InGaO3(ZnO)m (m is a natural number) belonging to homologous series is a known material (Non-Patent Documents 2 to 4).
In addition, it has been confirmed that such an In—Ga—Zn-based oxide can be used for a channel formation region of a thin film transistor (Patent Document 5, and Non-Patent Documents 5 and 6).